A parallel optical communications module is a module having multiple transmit (TX) channels, multiple receive (RX) channels, or both. A parallel optical transceiver module is an optical communications module that has multiple TX channels and multiple RX channels in the TX and RX portions, respectively, of the transceiver. The TX portion comprises components for transmitting data in the form of modulated optical signals over multiple optical waveguides, which are typically optical fibers. The TX portion includes a laser driver circuit and a plurality of laser diodes. The laser driver circuit outputs electrical signals to the laser diodes to modulate them. When the laser diodes are modulated, they output optical signals that have power levels corresponding to logic 1s and logic 0s. An optics system of the transceiver module focuses the optical signals produced by the laser diodes into the ends of respective transmit optical fibers held within a connector that mates with the transceiver module.
Typically, the TX portion also includes a plurality of monitor photodiodes that monitor the output power levels of the respective laser diodes and produce respective electrical feedback signals that are fed back to the transceiver controller. The transceiver controller processes the feedback signal to obtain respective average output power levels for the respective laser diodes. The transceiver controller outputs control signals to the laser driver circuit that cause it to adjust the modulation and/or bias current signals output to the respective laser diodes such that the average output power levels of the laser diodes are maintained at relatively constant levels.
The RX portion includes a plurality of receive photodiodes that receive incoming optical signals output from the ends of respective receive optical fibers held in the connector. The optics system of the transceiver module focuses the light that is output from the ends of the receive optical fibers onto the respective receive photodiodes. The receive photodiodes convert the incoming optical signals into electrical analog signals. An electrical detection circuit, such as a transimpedance amplifier (TIA), receives the electrical signals produced by the receive photodiodes and outputs corresponding amplified electrical signals, which are processed in the RX portion to recover the data.
There is an ever-increasing demand in the optical communications industry for optical communications systems that are capable of simultaneously transmitting and receiving ever-increasing amounts of data. To accomplish this, it is known to combine multiple parallel optical transceiver modules of the type described above to produce an optical communications system that has a higher bandwidth. A variety of parallel transceiver modules are used in the optical communications industry for this purpose. For example, one known transceiver module of the type described above includes a multi-fiber connector module known in the industry as the MTP® connector module. The MTP connector module plugs into a receptacle of a transceiver module that is secured to a front panel of the optical communications system. The MTP connector module receives a duplex fiber ribbon cable having a total of 4, 8, 12, 24, or 48 optical fibers. Typically, half of the fibers of the ribbon cable are transmit fibers and the other half are receive fibers, although all of the fibers may be either transmit or receive fibers in cases where the module is being used as either a transmitter or a receiver, but not both. When the MTP connector module is plugged into the receptacle, electrical contacts of the connector module are electrically connected with electrical contacts of a printed circuit board (PCB) of the transceiver module. The laser diodes and the photodiodes are integrated circuits (ICs) that are mounted on the PCB. A laser driver IC and a transceiver controller IC are typically also mounted on the PCB, although the transceiver controller IC is sometimes mounted on a separate IC, known as the motherboard IC of the optical communications system.
It is known that multiple transceiver modules of the type that use the MTP connector can be arranged in an array to provide an optical communications system that has an overall bandwidth that is generally equal to the sum of the bandwidths of the individual transceiver modules. One of the problems associated with such an array is that because the MTP connectors are edge-mounted in receptacles formed in the front panel of the optical communications system, there are limitations on the ability of such an array to achieve very large increases in bandwidth. For example, in order to obtain an optical communications system that has the ability to simultaneously transmit and receive one terabit of data per second (Tb/s), the racks and cabling needed to accommodate the transceiver modules would consume so much space that the solution would be impractical in many cases. In addition, an array of this type would present heat dissipation problems, and in most cases, would be prohibitively expensive.
An alternative to edge-mounting parallel optical transceiver modules is to mid-plane mount parallel optical transceiver modules. A mid-plane mounting configuration is one in which the modules are mounted in the plane of the motherboard PCB. One known parallel optical transceiver module that is mid-plane mounted is the Snap 12 transceiver module. The Snap 12 transceiver module comprises a 12-channel TX module and a 12-channel RX module. Each module has an array of 100 input/output (I/O) pins that plugs into a 100-pin ball grid array (BGA), known as a Meg-array. The Meg-array is, in turn, secured to the host PCB motherboard. The Snap 12 transceiver system has a bandwidth of 10 Gigabits (Gbs) per channel, and has a total bandwidth of 120 Gb/s.
The Snap 12 system is typically mounted in a box, which is connected to multiple electrical cables, which, in turn, are connected to multiple router ICs. In order to increase the total bandwidth of an optical communications system that uses multiple mid-plane mounted Snap 12 transceiver modules, multiple boxes may be used. The boxes are typically mounted in racks. For example, to obtain a system having a total bandwidth of ½ Tb/s, a total of five Snap 12 boxes would be needed. The racks needed to accommodate this many boxes and the cables needed to interconnect the boxes to the router ICs consume a large amount of space and generate a large amount of heat. The space consumption and heat generation problems must be dealt with in order to make the system operate properly.
In addition, the Snap 12 transmit and receive modules are relatively tall (approximately 15 mm in height), which often results in the occurrence of relatively large impedance disturbances in the modules. These impedance disturbances reduce signal integrity and therefore limit the bandwidth efficiency of the system. Also, each Snap 12 box is sold as a stand-alone part that is relatively expensive. Consequently, a system that is constructed of multiple boxes in order to achieve an increased bandwidth is generally very expensive.
Other mid-plane mounting solutions exist or have been proposed for mounting multiple parallel optical transceiver modules on a motherboard PCB. One of the problems associated with the existing or proposed mid-plane mounting solutions is that there are limitations on the mounting density of the modules on the motherboard PCB. One of the reasons for this is that the optical fiber ribbon cables that connect to the modules typically pass out of the side of the module that faces the front panel, which makes it necessary to provide some space between each module and the module behind it to avoid having to bend the ribbon cable of the module in front by a large amount to allow it to pass over the module in back of it. Consequently, the number of modules that can be mounted on the motherboard is limited by the additional space needed between adjacent modules to accommodate the cables.
In addition, with known edge-mounting and mid-plane mounting configurations, the electrical conductors that electrically connect the circuitry of the modules to their respective hub ICs on the motherboard PCB have to be relatively long. This is especially true with edge-mounted modules where the electrical conductors have to extend over a distance between the front panel where the modules are mounted and the hub IC. However, this is also true with mid-plane mounting configurations where the lengths of the conductors are increased due to the additional space that is needed between one module and the one behind it to accommodate the ribbon cable passing out of the side of the front module. Because these modules often operate at the limits of the latest high-speed electronics, the long conductor lengths tend to create signal integrity problems.
Accordingly, a need exists for an optical communications system having a mid-plane mounting configuration in which parallel optical transceiver modules are capable of being mounted with very high density on the motherboard PCB. Increasing the mounting density of the modules increases the amount of data that can be simultaneously transmitted from and received in the optical communications system. In addition, the high density of the modules on the motherboard enables the lengths of the conductors that connect the electrical circuitry of the modules to their respective hub ICs to be reduced, which helps prevent signal integrity problems.